MRI (Magnetic Resonance Imaging) is an imaging technique based on the phenomenon of nuclear magnetic resonance. Functional magnetic resonance imaging (fMRI) is a variation of magnetic resonance imaging which can be used for analysis and evaluation of brain function. Brain function consumes a large amount of energy and this is provided almost exclusively by glucose oxidation. Brain function is therefore dependent on glucose and oxygen, which is provided by the circulating blood. These characteristics have been exploited to study brain function in vivo.
The technique of functional MRI is based on the physiological principle that when nerve cells are active they consume oxygen carried by haemoglobin in red blood cells. In response the small blood vessels in the area dilate and blood flow is increased to the areas of increased neural activity. This delivers a large amount of oxygenated blood to the area.
The oxygen carried by the blood is bound to the protein haemoglobin. The magnetic properties of haemoglobin, oxygenated haemoglobin (oxyhaemoglobin) and deoxygenated haemoglobin (deoxyhaemoglobin) were demonstrated as long ago as 19361. The magnetic resonance (MR) signal of blood is modulated by the ratio of oxyhaemoglobin and deoxyhaemoglobin. Many decades later this property was shown in vivo and was termed “Blood Oxygen Level Dependent” (BOLD) contrast2. In BOLD fMRI changes in blood oxygen level are observed as signal changes from baseline.
In the BOLD method the fact that oxyhaemoglobin and deoxyhaemoglobin are magnetically different is exploited. Oxyhaemoglobin is diamagnetic whereas deoxyhaemoglobin is paramagnetic. As deoxyhaemoglobin is paramagnetic, it alters the T2* weighted magnetic resonance image signal. Thus, deoxyhaemoglobin is sometimes referred to as an endogenous contrast enhancing agent, and serves as the source of the signal for fMRI. The fMRI technique has been widely used for more than a decade to understand brain function by activating different brain areas by appropriate stimuli using different paradigms.
Upon neural activity, oxygen consumption is increased. This results in a corresponding reduction in deoxyhaemoglobin as the increase in blood flow brings more oxyhaemoglobin into the area without an increase of similar magnitude in oxygen consumption. This causes a small change in the magnetic field, and thus the MRI signal, in the active region. As deoxyhaemoglobin is paramagnetic, and the water molecules around the red blood cells are affected by the resulting local magnetic field distortions, a reduction of the T2* magnetic resonance image signal value is observed.
Despite the existence of such valuable imaging methods, there remains a need for improved techniques to permit better understanding of physiology, particularly to recognize metabolic dysfunction before it is too late for an appropriate intervention or procedure to be applied.
An object of the present invention is to provide an imaging technique intended to be useful in the study of viable tissue and organ function or dysfunction.